Stationary and transport applications of air conditioning and transport refrigeration systems control the temperature of a conditioned space to a predetermined temperature range adjacent to a predetermined set point temperature, with transport applications including those used with straight trucks, tractor-trailer combinations, refrigerated containers, and the like. Such air conditioning and refrigeration systems conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. The mechanical refrigeration cycle requires a refrigerant compressor driven by a prime mover, which often includes an internal combustion engine, such as a diesel engine. Because of the suspected depleting effect of CFC's on stratospheric ozone (O.sub.3), practical alternatives to the use of CFC's in air conditioning and refrigeration systems are being sought.
The use of a cryogen, i.e., a gas which has been compressed to a very cold liquid state, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2), in air conditioning and refrigeration systems is particularly attractive because, in addition to eliminating the need for a CFC, it also eliminates the need for a compressor and associated prime mover.
Refrigeration systems of which we are aware which utilize a cryogen, implement a cooling cycle by circulating the cryogen through a fluid flow path, or flow paths, of a heat exchanger which is in heat exchange relation with air from the space to be conditioned. When a heating cycle is required to hold a set point temperature, or to implement a defrost cycle, the cryogen is heated by a burner connected to a combustible fuel, and the heated cryogen is circulated through the fluid flow path, or flow paths of the conditioned space heat exchanger. Thus, cryogen is expended during both the cooling and heating cycles.
It would be desirable, and it is an object of the invention, to provide new and improved cryogenic based air conditioning and refrigeration systems, suitable for both transport and stationary applications, which more effectively and efficiently utilizes the cryogen, for lower cost operation, and for extending the operating time in a transport application for a given on-board supply vessel of cryogen.